It is apparent that when using -3db the image become cleaner with less visible noise. Likewise, using 3db there is more noise. (This is already known by most everyone, and I am not here to observe and point this out for the first time)

However, my question is, how relevant is -3db? Couldn't they have just called it 0db? If If the the engineers found an acceptable noise floor, and called it 0db, and then reduced the gain for a -3db setting, how much of this is just marketing?

If it 'really' is -3db, why didn't they make options to use a -6db and -9db, to be used in conjunction with the ND filters and to also provide an extremely clean crisp image under favorable lighting conditions? Woudn't this option provide an even better image?

While I cannot speak for Canon, in general, the gain is a measure of amplificatioin of the signal, not the light level. I would expect that the nominal "0" gain setting provides the optimum dynamic range for the system (in the designers opinion) and it provides the best compromise between shadows and highlights as read from an optimally exposed CCD. Any other gain setting will tend to "improve" the image shadows or highlights at the expense of the other end. (In film terms, adding gain is a bit like under exposing and then push processing.)

For example, at -3db gain, the CCD requires more light for the same mid-image output from the system. Depending on the scene lighting, this may means that the brightest highlights have lost some of their detail, but shadows will be cleaner because the signal off the deep shadow areass is further above the CCD noise floor